Humans can not exist without oxygen. In many environments, however, oxygen supply is insufficient or there is a risk of emergency situations involving a shortage of oxygen, for example in submarines, in mines, in space capsules, and also in air planes. Air pressure decreases with increasing flight altitude, and at cruising altitudes of many aircrafts, in particular long-range aircrafts, sufficient oxygen for human beings is no longer available. Therefore, the aircraft cabins are pressurized in order to ensure sufficient oxygen supply. In case of a sudden de-pressurization of an aircraft cabin, oxygen masks must be available, which supply oxygen to crew and passengers until the aircraft reaches a flight level where sufficient oxygen is available.
The oxygen which is provided by these emergency systems is typically produced by so-called “chlorate candles” or “oxygen candles”. These chemical oxygen generators contain chlorates or perchlorates as an oxygen source, as well as various additives such as fuels, catalysts, binders and moderators. Chlorate candles are often in the form of cylindrical rods, i.e. they have a shape similar to candles. Chlorate candles are disclosed, for example, in WO 97/43210.
While chlorate candles are widely used, they require high temperatures at which the oxygen production takes place. Namely, in chlorate candles oxygen is released at temperatures between 450° C. and 700° C. Therefore, effective heat insulation of chlorate candles is required, resulting in a weight and size penalty. Furthermore, decomposition of chlorates and perchlorates tends to produce toxic side products, in particular chlorine, which must be removed from the oxygen stream, thus additionally adding size and weight. Furthermore, there is a risk of system failure. In chlorate candles the reaction zone is normally liquid, i.e. there is a liquid zone travelling through the candle, starting at the point of ignition. The liquid zone within the otherwise solid candle considerably destabilizes the candle such that mechanical shocks or even slight vibrations may result in separation of the candle portions, thus interrupting the heat transfer and discontinuing the chlorate or perchlorate decomposition. In such a case, oxygen production may be interrupted, although oxygen is still vitally needed.
A different type of chemical oxygen generators uses peroxides as oxygen sources, for example sodium percarbonate, sodium perborate, or an urea adduct of hydrogen peroxide. Decomposition of the peroxides yields oxygen, and the decomposition reaction can be started by contacting the peroxide compounds with an appropriate enzyme or transition metal catalyst. Chemical oxygen generators of this type are disclosed in U.S. Pat. No. 2,035,896, WO 86/02063, JPS 61227903, and DE 196 02 149.
All these known peroxide-based oxygen generators have in common that they use water for providing contact between the peroxides and the catalysts. Unfortunately, water freezes at 0° C. and, therefore, no oxygen can be produced below 0° C. This is unacceptable for many emergency systems. An additional disadvantage of the aqueous systems is that the decomposition of peroxides in aqueous solutions results in vehement effervescing of the reaction mixture. As a consequence, an oxygen generating device containing a peroxide-based oxygen generating composition must have a complicated structure.
It would be beneficial to provide a solution to at least some of the problems of the prior art outlined above, and to provide a method for generating oxygen which produces breathable oxygen reliably and continuously in a wide temperature range, and preferably including subfreezing temperatures. The oxygen produced should be at a low temperature, preferably below 150° C., and further preferably free from toxic or otherwise noxious components such as chlorine or carbon monoxide. It would be also beneficial to provide a method capable to produce oxygen over an extended period of time and with a significant flow rate.